public class User {
private String username;
private String password;
// Getters and setters
}
public class Validator {
public static List<String> validate(Object obj) {
List<String> errors = new ArrayList<>();
Field[] fields = obj.getClass().getDeclaredFields();
for (Field field : fields) {
Annotation[] annotations = field.getAnnotations();
for (Annotation annotation : annotations) {
if (annotation instanceof NotNull) {
NotNull notNull = (NotNull) annotation;
try {
field.setAccessible(true);
Object value = field.get(obj);
if (value == null) {
errors.add(notNull.message());
}
} catch (IllegalAccessException e) {
e.printStackTrace();
}
} else if (annotation instanceof Size) {
Size size = (Size) annotation;
try {
field.setAccessible(true);
Object value = field.get(obj);
if (!(value instanceof String)) {
continue;
}
String string = (String) value;
errors.add(size.message());
}
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
}
}
return errors;
}
}
public class Main {
public static void main(String[] args) {
User user = new User();
user.setUsername(null);
user.setPassword("123");
List<String> errors = Validator.validate(user);
if (!errors.isEmpty()) {
for (String error : errors) {
System.out.println(error);
}
} else {
}
}
}
